<?php
$num1=$_GET["num1"];
$num2=$_GET["num2"];
$op=$_GET["op"];

switch($op)
{

    case'+':$result=$num1+$num2;break;
    case'-':$result=$num1-$num2;break;
    case'*':$result=$num1*$num2;break;
    case'%':$result=$num1%$num2;break;
}
?>

<html>
<body>
<form action="cal.php" method="get">
    <input type="text" name="num1" value="<?php echo $num1?>"/>
<select name="op">
    <option>+</option>
    <option>-</option>
    <option>*</option>
    <option>%</option>
</select>
    <input type="text" name="num2"  value="<?php echo $num2?>"/>=
    <input type="text" name="result"/>
    <button type="submit">计算</button>
</form>
</body>
</html>
